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DNA, RNA, Protein synthesis-from genes to proteins NUCLEIC ACIDS: Structure and Function DNA: Where the genetic information is stored, blueprint for making proteins. Can be copied and passed from generation to generation RNA: Always involved in protein synthesis. It is produced every time proteins are needed 2 Cytosine Nitrogenous base Macromolecules (polymers!) Monomers (units): nucleotides Adenine Guanine Phosphate group Ribose (RNA) 5-carbon sugar Deoxyribose (DNA) Thymine DNA DNA Uracil Uracil RNA “Accepted” as the heritable material in 1952 TWO strands with helical disposition. One strand is the code, the other helps protecting the coding strand backbone: sugar and phosphate sequence. Both N-base and phosphate connect to the sugar Nitrogen bases (N-bases) 1) Hydrogen bonds between them attach one strand to the other. 2) arranged in a specific “format”, always is 3 H bonds: Cytosine C G Guanine 2 H bonds: Adenine A T Thymine N-bases store the genetic information! 1 DNA, RNA, Protein synthesis-from genes to proteins DNA Replication When a cell reproduces, a complete copy of the DNA must pass from one generation to the next. Occurs (in the nucleus) during Interphase, in preparation for cell division 2 (1) An enzyme breaks H bonds between nitrogenous bases (2) Strands are separated and both are copied (two directional copy!) (3) Enzyme sequentially attaches new nucleotides with dehydration reactions (4) Replication bubbles combine and two new strands complementary to the original ones are formed DNA Replication: Is done by the enzyme DNA polymerase (1) Breaking of H bonds by enzymes (2) Strands are separated (3) Enzymes copy in both directions New nucleotides are added by the enzyme (4) Combination of the different replication sites 2 DNA, RNA, Protein synthesis-from genes to proteins Mutations • Are any change in the nucleotide sequence of DNA Some mutations result in changes in the final protein 4 types of mutations 1) Missense: One nucleotide changes. Only one amino acid is changed 2 2) Nonsense: One nucleotide changes. Produces a STOP codon and shorter protein as a result 3) Frameshift: By either inserting or deleting 1 or 2 letters the entire sequence is changed after the mutation, which adds a bunch of wrong amino acids to the protein 4) Silent mutations: One nucleotide changes, but the amino acid that is being coded for is the SAME! Mutation happen, but the new codon codifies for the same amino acid DNA changes but protein is the same Hemoglobin: 145 amino acids (aa) in four chains. A missense mutation (one nucleotide) changes one aa • hemoglobin shape changes when oxygen levels are low •Irregular shape causes hemoglobin to clump, blocking the blood vessels • no oxygen delivering weakness, brain damage, rheumatism In which cells is a mutation potentially more dangerous? Sex cells! Cells used for reproduction THE FLOW OF GENETIC INFORMATION FROM DNA TO RNA TO PROTEIN Set of ideas describing how the cell uses the information stored in the DNA Makes a copy of DNA in the form of RNA Synthesizes proteins using mRNA as template Structural Carrier, Enzymes, Hormones, antibodies 3 DNA, RNA, Protein synthesis-from genes to proteins RNA • ONE strand • backbone: sugar and phosphate sequence • bases: arranged in a specific “format”, always 2 Cytosine C G Guanine Adenine A U Uracil Three types of RNA: all copies of parts of DNA strands 1) mRNA: “messenger” RNA, carries the genetic information from the cell nucleus to the cytoplasm 2) tRNA: “transfer”RNA, transfers or carries amino acids to the ribosomes, and aids in the pairing of amino acids 3) rRNA: “ribosomal” RNA, covered in protein to form a ribosome, where the protein synthesis (translation) takes place Transcription DNA • DNA to RNA • Takes place in the nucleus tRNA rRNA (ribosomes) transcription mRNA Proteins translation Why a copy? (1) Every chromosome ranges between hundreds to thousands of genes, so it is impractical to move out the entire chromosome when only one gene is actually needed (2) The nucleus would have to be destroyed every time a chromosome has to move out! (3) Also, in this way the original “library” remains safe, no risks of getting the DNA damaged or destroyed 4 DNA, RNA, Protein synthesis-from genes to proteins transcription DNA • DNA to RNA • Takes place in the nucleus tRNA rRNA (ribosomes) mRNA Proteins translation RNA polymerase •The two strands of DNA are separated and the bases of the coding strand exposed 2 Nucleotides Available in the nucleus •The coding strand is read in one direction (Only the “coding strand is used in this case) •Dehydration synthesis reactions bonding of nucleotides and construction of a single strand of RNA mRNA, carries the code or “message” on how to make a protein Controlling Transcription… Which strand of DNA must be used for transcription? When to stop making RNA? Where must transcription (RNA generation) begin? rr rr Promoter Sequence recognized by the RNA polymerase The “start transcribing” signal is a nucleotide sequence called a promoter. The first phase of transcription is initiation, in which: RNA polymerase attaches to the promoter RNA synthesis begins rr Gene rr Termination region Marks the end of transcription During the third phase of transcription, called termination: RNA polymerase reaches a sequence of DNA bases called a terminator RNA Polymerase detaches from the RNA The DNA strands rejoin Regulatory Regions (rr): Specific proteins called transcription factors attach to these parts making the promoter region more o less “visible” to the RNA polymerase 5 DNA, RNA, Protein synthesis-from genes to proteins Translation tRNA rRNA (ribosomes) transcription DNA • RNA language to protein language • Using a mRNA template to synthesize proteins mRNA How does the cell machinery know which amino acid goes with each part of RNA? Proteins translation 2 tRNA is the translator! mRNA is the dictionary! The code in the mRNA is used to direct the sequential assembly of amino acids in the cytoplasm tRNA Each sequence of 3 nitrogenous bases (called codon) in the mRNA defines an amino acid Translates the code to the amino acid language and transports the aminoacids The tRNA has bases (named anticodon) that match the codon of the mRNA with the correct amino acid Codon mRNA (3 bases code in mRNA) UGU = Cystine Translation: the players… Translation requires: Messenger RNA (mRNA), bring the code on how to make the protein Transfer RNA (tRNA), works as the “translator” from Nucleic acid language to protein language Amino acids to build the Next aminoacid protein to be added Ribosomes as the “factory” where the entire process takes place Growing polypeptide (protein) Ribosome tRNA Anti codon mRNA Codons 6 DNA, RNA, Protein synthesis-from genes to proteins Where to begin and end translation? The “signal” is present in the mRNA! STOP! 3 specific codons are used to indicate STOP! Termination of the translation process 2 Codon (3 bases code in mRNA) UGU = Cystine START! Only one codon is the initiator Start of the translation process The process of translation (protein synthesis) A large ribosomal subunit binds, creating a functional ribosome 1 Initiation 2 3 mRNA molecule binds to a small ribosomal subunit initiator tRNA binds to the start codon A new tRNA with an amino acid approximates to the ribosome 6 When a stop codon is found in the code translation stops tRNA, ribosomes, and enzymes can be reused mRNA is broken down An enzyme Elongation 8 4 The first tRNA detaches from the amino acid and leaves ribosome The mRNA moves catalyzes peptide formation 5 7 DNA, RNA, Protein synthesis-from genes to proteins How fast can cells synthesize proteins? 2 Average protein synthesis time • Prokaryotes (~6 sec) – ~300 amino acids – Translation rate (~50 aa/sec) • Eukaryotes (~2 min) – ~500 amino acids – Translation rate (5 aa/sec) Fedorov and Baldwin (1997) J. Biol. Chem. 272:32715 8